1. Field of the Invention
This invention relates to an improved process for the isomerization of ortho- and meta-xylenes to para-xylene, as well as removal of ethylbenzene from the C.sub.8 aromatics stream.
2. Description of Related Art
Catalytic reforming hydrocarbon feeds such as naphtha using conventional aromatization catalysts produces reformate which is richer in the content Of C.sub.6 -C.sub.10 aromatics than the feeds. Of these aromatics, significant quantities of C.sub.8 aromatics are produced which comprise a mixture of ethyl benzene, and mixed ortho-, meta- and para-xylene isomers. Typically, the product from the catalytic reformer (reformate) is fed to an aromatic extraction plant where the aromatics, e.g. C.sub.6, C.sub.7 and C.sub.8 aromatics, are separated from the paraffins and other non-aromatic products present in the reformate. The C.sub.8 aromatic fraction may then be separated from the lower boiling C.sub.6 and C.sub.7 aromatics by distillation.
The C.sub.8 aromatic fraction normally contains a mixture of ethyl benzene and the ortho, para and meta xylene isomers. The three xylene isomers are usually present in near thermodynamic equilibrium amounts, e.g., generally 52-53 wt % meta xylene, 23-24 wt.% para xylene and 23.5 to 24.5 wt % ortho xylene. Because para-xylene is a more valuable isomer used as a chemical intermediate in a number of applications, it may be desirable to separate the para-xylene from the other isomers using conventional techniques such as crystallization, or by adsorption/desorption on zeolites. After such separation, the residual C.sub.8 aromatic fraction contains non-equilibrium quantities of ethyl benzene and the mixed ortho and meta-xylene isomers and is lean with respect to para-xylene content.
The para-xylene lean residual product may be further upgraded by subjecting it to isomerization conditions wherein at least a portion of the ethyl benzene undergoes a hydrogenolysis reaction to benzene and ethane and a portion of the ortho and meta xylenes are isomerized to produce a mixture which once again approximates the equilibrium concentration of the ortho, meta and para xylene isomers. Typically such isomerization conditions comprise contacting the non-equilibrium C.sub.8 aromatic feed with a suitable isomerization catalyst, such as a molecular sieve, in a suitable reactor at temperatures above about 700.degree. F. and at pressures sufficient to maintain the reaction in the vapor phase.
Numerous catalysts have been proposed for use in the isomerization process. For example, U.S. Pat. No. 4,331,822 discloses vapor phase isomerization in the presence of added hydrogen using a crystalline aluminosilicate zeolite such as ZSM-5 wherein the catalyst contains two different metals, one being platinum and the other being a metal inclusive of metals such as zinc. However, a disadvantage associated with the use of noble metals such as platinum in such processes, besides the expense of such metals, is that use of the platinum loaded catalyst can lead to a substantial loss of xylenes during the isomerization reaction as a consequence of acid-catalyzed trans-alkylation reactions and ring cracking reactions. Also, platinum-containing catalysts require a more complicated and time consuming regeneration process after they are at least partially deactivated as the result of coke build-up on the catalyst surface.
Another isomerization process disclosed in U.S. Pat. No. 4,584,423 involves the use of a zeolite catalyst such as ZSM-5 which is loaded with 0.05 up to 1.5 weight percent of a metal selected from the group consisting of zinc, cadmium, iron, barium, tin and cesium. The process is conducted in the absence of added hydrogen and at relatively low pressures below about 100 psig. The process is demonstrated to produce relatively low xylene loss in the range of about 1.21 to about 2.65 percent at a relatively high ethyl benzene conversion rate.
However, a disadvantage of such a process conducted without the use of added hydrogen and at relatively low pressure is that the catalyst tends to more quickly deactivate due to coke build-up thereby shortening run lengths.